As it is known, weft feeders for textile machines comprise a stationary drum on which a plurality of yarn loops forming a weft reserve are wound. Upon request from the loom, the loops are unwound from the drum, then pass through a braking device which controls the tension of the yarn, and finally feed the loom.
In the weft feeders of the above kind, which are known from prior art documents in the name of the present Applicant, such as EP 1 059 375, the braking device typically comprises a frustoconical hollow member which is supported at the centre of an annular support on a spider assembly of springs, and is biased with its inner surface against the end of the drum from which the loops are unwound. A pair of linear actuators operatively connected to the annular support are driven by a control unit having a position control loop and a current control loop, which is capable of generating a modulated current as a function of the fluctuations of the yarn tension, in order to modulate the pressure applied upon the drum by the cone. This assembly is supported on a slide that is longitudinally movable under control of a worm screw mechanism that is manually operable in order to adjust the static pressure, or preload, applied upon the drum by the cone at rest. Therefore, the unwinding yarn runs between the drum and the frustoconical member, which modulately applies the desired braking action upon the yarn.
Although the above control unit allows the braking action to be modulated smoothly and dynamically, however it has the drawback that its accuracy considerably decreases when certain parameters are changed, such as the stiffness of the springs which support the frustoconical member, or the static pressure applied upon the drum by the cone, which parameters are chosen, e.g., on the basis of the type of yarn under processing, the loom speed, the loom height, and the like. In fact, as well known to the person skilled in the art, the position control loop is designed to operate accurately with a specific set of springs and with a predetermined value of preload. On the contrary, changing these parameters results in an error of compensation. The more said parameters differ from the design parameters, the more relevant said error.